1. Field of the Invention
The present invention relates to semiconductor devices and, in particular, refers to an optoelectronic device.
2. Description of the Related Art
Photodiodes are used in optical communication networks to convert optical signals to electrical signals for processing using electronic circuits. In high bandwidth applications, the photodiodes can often be packaged using flip chip interconnections. The flip chip interconnections provide many benefits, including smaller package size, elimination of bond wires, and improved signal integrity.
FIG. 1 illustrates a typical flip chip package for a backside-illuminated photodiode. A photodiode chip is placed face down on a chip carrier 100. Solder bumps 102 couple respective contacts 104 on the front side 106 of the photodiode chip to corresponding contacts 108 on the chip carrier 100. The contacts 104 on the front side 106 of the photodiode chip are electrically coupled to a photodiode device 110 formed on the front side 106 of the photodiode chip. In the particular application illustrated in FIG. 1, light signals 112 from a fiber optic cable (not shown) are incident on a backside 114 of the photodiode chip and travel through a chip substrate 116 before reaching the photodiode device 110 on the front side 106 of the photodiode chip.
The design of a backside illuminated photodiode is complex, and its performance is usually inferior to a front side illuminated photodiode. For example, the thickness of the chip substrate 116 may range from 100 micrometer (μm) to 20 μm. Light exiting from the fiber optic cable may diverge causing the light spot to get bigger when it reaches an active detecting area of the photodiode device 110 on the front side 106 of the photodiode chip. A photodiode with a wider aperture may be required to capture all of the light, which will increase the device capacitance and degrade the speed of the device. The divergence of the light beams may also require an additional lens on the backside 114 to re-focus the light signal 112.
In addition, the fiber optic cable coming to the backside 114 usually needs active alignment, which properly positions the fiber optic cable with respect to the photodiode device 110 based on signal performance. The normal method of attaching the fiber optic cable with minimal back-reflection is to cut the fiber optic cable at some small angle so that reflected light does not couple back to the fiber optic cable. An angled light signal coupled with a thick chip substrate requires active alignment of fiber to the photodiode since the alignment tolerance is very small. Active alignment is time consuming and an expensive assembly technique.
Furthermore, testing a backside-illuminated photodiode is not as easy as testing a front illuminated photodiode. Responsivity, linearity, and speed tests are difficult to perform when the photodiode is in wafer form since probe pads are on the front side 106 of the chip while the light signal 112 has to come in from the backside 114. Sampling tests are done after the chip is mounted in a flip chip package, which adds cost and long cycle time to the testing of the backside illuminated photodiode. Finally, backside illuminated photodiodes are not feasible using absorptive chip substrates, such as, for example, silicon (Si), germanium (Ge) or gallium arsenide (GaAs) semiconductors.
FIG. 2 illustrates another flip chip package which allows for front side illumination of the photodiode device 110. FIG. 2 is substantially similar to FIG. 1, except a hole is cut through the chip carrier 100 to allow light to illuminate directly on the photodiode device 10 formed on the front side 106 of the photodiode chip. Although the flip chip package shown in FIG. 2 alleviates some of the problems associated with backside illumination, the hole through the chip carrier 100 imposes more difficulties in the packaging of the photodiode and is not conducive to chip-on-chip packages.